1. Field of the Invention
The present invention relates to photocatalytic material which is capable of exhibiting photocatalytic activity when irradiated with visible and ultraviolet light, a photocatalyst, a photocatalytic article and a method for the preparation thereof.
2. Description of the Background
Conventionally known materials exhibiting a photocatalytic action include TiO2 (titanium dioxide), CdS (cadmium sulfide), WO3 (tungsten trioxide), and ZnO (zinc oxide), for example. These photocatalytic materials are semiconductors, absorb light to form electrons and holes, and also promote various chemical reactions and bactericidal actions. However, because titanium oxide is nontoxic and exhibits a superior stability to water and acid, thus far, only titanium oxide has afforded practical commercial use as a photocatalyst.
However, because of the values of the band gap (Eg=3.2 eV) of titanium oxide the operating light of such a titanium oxide photocatalyst is limited to ultraviolet light with a wavelength xcex less than 380 nm. As a consequence, an unfulfilled demand exits for materials which exhibit catalytic activity when irradiated with visible light having a wavelength of 380 nm or longer. These materials are desired, for example, for use indoors and for improving photocatalytic activity.
As described in Japanese Patent Laid-Open publication No. Hei 9-262482, by modifying materials using ion implanting of metal elements such as Cr (chromium) and V (vanadium) in anatase type titanium oxide having a high catalytic activity, the light absorbing edge of titanium oxide can be shifted to the long wavelength side to permit the operation of titanium oxide catalyst in visible light. Although a number of reports discussing the doping of Cr, and V, for example, on have been published since the early 1970s, these reports describe, however, that in instances where operation under visible light is enabled, the performance of the titanium oxide sharply lowers.
On the other hand, as described in Japanese Patent Laid-Open publication No. Hei 9-262482, the original performance of titanium oxide can be maintained through use of special techniques for doping Cr, and V, for example.
Thus, in the above conventional example, the operation of titanium oxide photocatalyst under visible light is made possible by ion implantation of metal elements in titanium oxide. However, metal ion implantation is disadvantageous because of the high cost thereof. While there is a demand for other methods of manufacturing and synthesizing of TiO2 photocatalyst, such as synthesis in solution or sputtering, photocatalysts produced through these methods still can not operate under visible light. It is generally considered that this is because Cr of the dopant aggregates or forms oxides, such as Cr2O3, in a crystallization process. Thus, the technique of ion implantation must be adapted in order for metal elements to be used to enable the operability of titanium oxide under visible light. Unfortunately, this method entails an expensive large-scale apparatus and a very high manufacturing cost. While attempts have been made to realize visible light operation by doping trivalent elements, the optimum state of such doping has not yet been realized.
Furthermore, Japanese Patent Laid-Open publication No. 2000-140636 opened to the public after the filing of the two applications from which the present application claims priority, discloses a method of forming a photocatalytic substance by doping trivalent elements in titanium oxide and that one of aluminum, boron, and nitrogen is used as the trivalent element. The example given in this publication describes that acetaldehyde may be decomposed more efficiently in a powder body in which titanium oxide and its nitrogen dope body are laminated in many layers, than in a titanium oxide powder, when light is irradiated from a natural light fluorescent lamp. However, the titanium oxide in that example is prepared by doping trivalent elements in which nitrogen is merely doped or unintentionally mixed and the photocatalyst is not constructed so as to maximize photocatalytic activity under visible light. In addition, in the example of that publication wherein a fluorescent lamp is employed, a description of the effects of ultraviolet light radiated by a fluorescent lamp is included. An experimental method employing a fluorescent lamp, even one designated as natural light, is not suitable for evaluating activity under visible light because any observed improvement in photocatalytic activity might be attributed to enhanced UV activity owing to structural changes, such as an increased surface area or the like. Therefore, it is likely that the true photocatalytic activity under visible light of the nitrogen doped body of this titanium oxide is not as great as indicated by the example as the observed increase in photocatalytic activity may be due to enhanced UV activity for the reasons mentioned.
Thus, a need exists for a titanium oxide photocatalyst having visible light absorption, and which can be produced, if desired, without using ion implantation.
Accordingly, it is an object of the present invention to provide a titanium oxide photocatalyst having visible light adsorption and, moreover, to provide a photocatalyst which has higher visible light absorbing efficiency without using ion implantation.
It is also an object of the present invention to provide a photocatalytic composition.
It is, further, an object of the present invention to provide a photocatalytic article.
The above objects and others are provided by a photocatalytic material, which exhibits photocatalytic activity when exposed to light containing a wavelength in a region of visible light, the material containing Tixe2x80x94Oxe2x80x94N containing nitrogen in lattices of titanium oxide crystal.